In fuel cells, electrical energy is produced by reacting a fuel with an oxidant in the presence of a catalyst. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H.sup.+) in acid electrolytes, or the hydroxyl ion (OH.sup.-) in alkaline electrolytes. At the anode, incoming hydrogen gas ionizes to produce hydrogen ions and electrons. Since the electrolyte is not an electronic conductor, the electrons flow away from the anode via the external circuit. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct, which is then typically extracted as a vapor. One well-known type of fuel cell includes a "membrane-electrode-assembly" (MEA) which is typically a thin, proton-transmissive, solid polymer membrane electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017. In practice, a number of these unit fuel cells are normally stacked or `ganged` together to form a fuel cell stack or assembly. The individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. Degradation or failure of only a single one of the unit cells in the stack lowers the overall performance of the fuel cell assembly, and may completely disable it. Fuel cells fail for a number of reasons including carbon monoxide poisoning of the catalyst, flooding of the cells with water, and leakage of gaseous hydrogen around or through the proton exchange membrane. Transport of gaseous hydrogen to the cathode side of the membrane can result in non-useful hydrogen consumption, loss of cell/stack efficiencies and a drop in cell voltage. Carbon monoxide poisoning and/or water flooding result in a drop in the unit cell and/or stack voltage. When any of these situations are indicated, corrective action is warranted to prevent irreversible cell/stack degradation. If one of the membranes in the stack degrades or malfunctions, the entire stack must be removed and disassembled in order to repair the cell. In the case of stack designs where welding and/or adhesive are used for assembly, the entire stack may need to be discarded. In addition, the inner cells of the stack do not operate at the same efficiency as the outer cells of the stack due to differences in temperature and humidity between the outside and inside layers.
It would be highly desirable if there were a way to monitor the performance of the individual cells during operation of the fuel cell assembly and then to adjust the operation of the fuel cell stack to either optimize it in response to various environmental conditions or to compensate for a degraded unit cell. Some in the prior art have attempted to solve this problem by electrically interrupting the cell from operation for a few milliseconds and measuring current and voltage of the cell at some fixed point. This method has the disadvantage of requiring a complicated switching scheme. The cell has to be switched to a test load, measured, then switched back. In addition, the information is limited to a single point or at most, a few points on the current-voltage (IV) curve, which tells little about the condition of the actual cell.